Drosophila melanogaster males show a progressive loss of fertility with age. This reduction in fertility is not strongly linked to sperm production but is due primarily to a progressive loss in the ability of aging males to inseminate females. This decline in insemination rate probably results from decreasing sexual drive and failure to transfer sperm during copulation. Failure to transfer sperm likely results from a rapidly diminishing ability to synthesize the protein components of the seminal fluid required for effective sperm transfer. Declining sexual drive may be linked to adult levels of juvenile hormone which influence the activity of esterase 6 (EST 6). This enzyme is localized in the anterior ejaculatory duct of the male reproductive system and is depleted during mating as a result of its transfer to females as a component of the seminal fluid. The relationship between male reproductive decline, the synthesis of seminal-fluid proteins, and sexual drive will be explored by (1) determining the rates of synthesis and turnover of EST 6, (2) measuring the amounts of seminal fluid components transferred to females by aging males, (3) determining the kinetic parameters of EST 6 isolated from flies of differing ages, and (4) measuring age-related changes in mating speed in untreated males and males supplied with exogenous hormone. Possible age-related changes in effective hormone levels will be assessed by use of an in vitro organ culture system. Preliminary results suggest that changes in the rates of synthesis of seminal fluid proteins with male age are controlled by DNA-level mechanisms. This possibility will be investigated by using recombinant-DNA techniques to clone the Est 6 gene which can then be used to measure rates of messenger RNA synthesis. Previous studies and the proposed investigations will demonstrate that male fertility loss and related biochemical changes can serve as a model system in which the underlying biochemical mechanisms responsible for a clearly defined aging process can be elucidated.